The work is devoted to the problem of electromagnetic field excitation by a point charge moving along the axis of a cylindrical waveguide. The waveguide consists of a vacuum part and a part having cylindrical dielectric layer. The charge flies from the partially dielectric part into the vacuum one. Investigation is carried out by division of the total field into the “forced field”, i.e., the field of the point charge in the infinite regular waveguide, and the “free field” which describes the influence of transverse boundary. The solution is constructed using the expansion of the free field into a series of waveguides modes in both parts of the structure. The infinite set of equations is obtained for the modes excitation coefficients. The analytical and numerical analysis of the total field is conducted. Special attention is devoted to the so-called “Cherenkov-transition radiation” (the effect of exit of Cherenkov radiation into the vacuum part of the waveguide). Distribution of radiation power on frequencies and waveguide modes is analysed. It is shown that the CTR can be both multi-frequency radiation and practically monochromatic one (depending on the problem parameters).